The present invention relates to a substantially odorless solvent for the chromogenic dye-precursor material for a pressure-sensitive recording paper sheet and a pressure-sensitive recording paper sheet prepared by using the solvent. More in detail, the present invention relates to a solvent for the chromogenic dye-precursor material for the pressure-sensitive recording paper sheet, which comprises from 0 to 20% by weight of m-isopropylbiphenyl, from 40 to 75% by weight of p-isopropylbiphenyl and from 5 to 40% by weight of diisopropylbiphenyl, and a pressure-sensitive recording paper sheet comprising a colour-development sheet coated with microcapsules containing a dye solution which is composed of the chromogenic dye-precursor material and the substantially odorless solvent, and a colour-developer sheet.
A pressure-sensitive recording sheet comprises a colour-development sheet prepared by coating microcapsules, in which a solution of a colourless electron-donating chromogenic dye-precursor material having a colouring reactivity has been encapsulated, onto a supporting sheet and a colour-developer sheet prepared by coating a colour-developer which develops a colour on contacting to the chromogenic dye-precursor material onto the supporting sheet.
In recent years, the pressure-sensitive recording paper sheets have been broadly used instead of carbon copying paper sheets and back-carbon copying paper sheets of pigment type.
As the pressure-sensitive recording paper sheets, it is necessary that they are excellent in colour-development, stability for long term preservation and light resistance and that they are low in toxicity not to cause environmental pollution.
As the solvent which dissolves the chromogenic dye-precursor material for the pressure-sensitive recording paper sheet, it is demanded that the solvent fulfills the following requirements.
(1) To dissolve the chromogenic dye-precursor material to a high concentration.
(2) Not to cause the decomposition and colour-development of the chromogenic dye-precursor material.
(3) To show a considerably high boiling point, and not to evaporate in the thermal drying step and under a high atmospheric temperature.
(4) Not to be educed to water on encapsulating.
(5) To show a high speed of colour-development and a high concentration of the developed colour as well as the high colour stability after colour-developing.
(6) To be stable to light, heat and chemicals.
(7) To show a low viscosity so that its flow out from the broken capsules is freely carried out.
(8) To be substantially odorless.
(9) To show a low toxicity to human body and to be safe.
(10) To show a favorable biodegradability and not to cause environmental pollution.
As the pressure-sensitive recording paper sheet prepared by using the solvent for the chromogenic dye-precursor material, which fulfills the above-mentioned requirements, the following examples have been proposed.
A pressure-sensitive recording paper sheet prepared by using a solvent comprising at least one of alkylated biphenyls or alkylated terphenyls having alkyl group(s) of from 1 to 12 carbon atoms or the mixture with another solvent, wherein the number of the alkyl groups in the alkylated biphenyl is from 1 to 4 and the number of the alkyl groups in the alkylated terphenyl is from 1 to 6, with the proviso that not less than two alkyl groups are the same or different from each other [refer to Japanese Patent Publication No. 49-21608 (1974)].
A material for a recording paper sheet coated with microcapsules encapsulating a solution of a chromogenic dye-precursor material in a solvent comprising not less than 65% by weight of isopropylbiphenyl represented by the formula: ##STR1## not more than 25% by weight of polyisopropylbiphenyl and not more than 10% by weight of biphenyl [refer to Japanese Patent Publication No. 54-37528 (1979)].
Further, as the method for producing an alkylbiphenyl, a method for producing an alkylbiphenyl rich in m- and p-substituted substances has been known, wherein from 0.5 to 5 mol of a lower olefin is brought into contact with 1 mol of biphenyl at a temperature of not less than 180.degree. C. in the presence of from 0.5 to 10% by weight of a silica-alumina catalyst, a zeolite catalyst or a mixture thereof [refer to Japanese Patent Application Laid-Open No. 56-156222 (1981)].
Particularly, isopropylbiphenyl which is the main component constituting the solvent for the chromogenic dye-precursor for the pressure-sensitive recording paper sheet disclosed in the Japanese Patent Publication No. 54-37528 (1979) is an isomeric mixture of the compound represented by the following structural formula: ##STR2## wherein the isopropyl group is bonded to the benzene ring at the position of ortho, meta or para thereof.
Further, in Japanese Patent Publication No. 54-37528 (1979), it has been shown that the suitable composition of the solvent for the chromogenic dye-precursor material for a pressure-sensitive recording paper sheet comprises about 50% by weight of m-isopropylbiphenyl, 30% by weight of p-isopropylbiphenyl, 25% by weight of polyisopropylbiphenyl and 0.5% by weight of biphenyl.
Still more, the commerciallized "isopropylbiphenyl" and the isopropylbiphenyl synthesized by Friedel-Crafts alkylation of biphenyl (refer to Industrial and Engineering Chemistry Product Research and Development Vol. 8, 239-241, 1969) is a mixture of m-isomer and p-isomer (the weight ratio of m-isomer to p-isomer is about 2:1) and contains a little amount of o-isomer.
With the propagation of the pressure-sensitive recording paper sheets, the case where the pressure-sensitive recording paper sheets are used in cold districts of not more than 0.degree. C. in the ambient temperature or transported and preserved for a long time in the environment of about from 40.degree. to 50.degree. C. in the ambient temperature and of not less than about 80% in the relative humidity has increased.
Particularly, in the outdoor facilities such as gasoline service stands, the pressure-sensitive recording paper sheets are used under the environment of not more than 0.degree. C. in winter.
Since in such an environment of low atmospheric temperatures, (1) the solvent for the chromogenic dye-precursor material used in the pressure-sensitive recording paper sheet crystallizes, (2) it takes a very long time for obtaining clear development of colour or (3) the colour-developed image is very light in colour, if developed, not to be deciphered, such a pressure-sensitive recording paper sheet is not to be put to practical use. Namely, it is demanded that an initial colour developing activity within about 30 sec from the recording is at least 40%.
Furthermore, from the viewpoint of the production and preservation of the pressure-sensitive recording paper sheet, an offer of a substantially odorless solvent for the chromogenic dye-precursor material for the pressure-sensitive recording paper sheet has been earnestly desired. The problem of offensive odor occurs in the process for preparing the pressure-sensitive recording paper sheet when the recording paper sheet material prepared by applying the microcapsules containing the solution of the chromogenic dye-precursor material onto the supporting paper sheet is cut after drying thereof. Namely, when the pressure-sensitive recording paper sheet is cut at a relatively high temperature of from 40.degree. to 50.degree. C., the solvent flowing out from the thus broken microcapsules gives a disagreeable impression to the operators. In addition, in the cases when the solvent adheres to clothes or hands of the operator in the cutting step of the pressure-sensitive recording paper sheet or in handling of the solvent, the odor still remains even after washing the clothes or the hands with a cleanser to give a disagreeable impression. Furthermore, the pressure-sensitive recording paper sheets which have been subjected to recording are assembled and preserved in a storehouse for a relatively long time. In such an occasion, the odor emitted from a large amount of the pressure-sensitive recording paper sheets which have been subjected to recording becomes the cause of disagreeable impression.
On examining the above-mentioned problem of odor of the pressure-sensitive recording paper sheet, the isopropylbiphenyl which has been hitherto proposed as the solvent for the chromogenic dye-precursor material for the pressure-sensitive recording paper sheet is not favorable as the solvent for such a purpose. Namely, the isopropylbiphenyl is an isomeric mixture of o-isopropylbiphenyl, m-isopropylbiphenyl and p-isopropylbiphenyl or an isomeric mixture of m-isopropylbiphenyl as the main component and p-isopropylbiphenyl. Ortho-isopropylbiphenyl shows a disagreeable odor, and also m-isopropylbiphenyl has an odor. On the other hand, although p-isopropylbiphenyl is almost odorless and excellent in the property of dissolving the chromogenic dye-precursor material, since the melting point of p-isopropylbiphenyl is relatively high, it has a defect that crystals thereof are apt to be educed at a low temperature.
Accordingly, it has been desired to provide with a solvent for the chromogenic dye-precursor material for the pressure-sensitive recording paper sheet, which is almost odorless, from which crystals thereof are scarcely educed at a low temperature (for instance, 0.degree. C.) and which shows an excellent initial colour-developing property.
As a result of the present inventors' studies for providing with a solvent for the chromogenic dye-precursor material for a pressure-sensitive recording paper sheet, which has the specific character required as above, it has been found out by the present inventors that a solvent comprising from 0 to 20% by weight of m-isopropylbiphenyl, from 40 to 75% by weight of p-isopropylbiphenyl and from 5 to 40% by weight of diisopropylbiphenyl is almost odorless, that the crystals of such a solvent are scarcely educed at a low temperature and that such a solvent fulfills all the conditions to be possessed by the solvent for the chromogenic dye-precursor material for a pressure-sensitive recording paper sheet, and on the basis of the findings, the present inventors have effected the present invention.